Recently, white light emitting diodes (white LEDs) having a combination of a light emitting diode (LED) and a phosphor are employed as white light sources for backlight sources and lighting devices of displays. Among them, white LEDs using InGaN based blue LEDs as light emitting diodes are widely used.
A phosphor used for white LEDs must be efficiently excited by emission of a blue LED to emit fluorescence of visible light. A representative example of such a phosphor includes an yttrium aluminum garnet (YAG) phosphor activated by Ce, which is efficiently excited by blue light to exhibit broad yellow light emission. Combination of a single YAG phosphor with the blue LED can provide light emission in a wide range of visible light together with pseudo white color. Therefore, the white LEDs containing the YAG phosphor are used for lighting and backlight sources.
However, the white LEDs containing the YAG phosphor have a problem that they exhibit both a lower color rendering property for a lighting application and a narrower color reproduction range for a backlight application, because of a lower amount of red components.
Therefore, white LEDs with an improved color rendering property and color reproducibility are developed, which have a combination of a red phosphor that can be excited by a blue LED; a YAG phosphor; and a green phosphor such as β-sialon activated by Eu and orthosilicates. For the red phosphor used for such white LEDs, nitride or oxynitride phosphors having Eu2+ as a luminescent center are often used, because the phosphors exhibit a higher fluorescence conversion efficiency, lower luminance reduction at elevated temperature, and good chemical stability. Representative examples of the nitride and oxynitride phosphors include those represented by chemical formulae: Sr2Si5N8:Eu2+, CaAlSiN3:Eu2+, and (Ca,Sr)AlSiN3:Eu2+.
However, the red phosphor having Eu2+ as a luminescent center has a problem that the red phosphor results in significantly decreased luminance of the white LED as compared with the case of the YAG phosphor alone, despite the red phosphor exhibits a higher fluorescence conversion efficiency, because the red phosphor has a broad emission spectrum and contains many luminescent components with low luminosity factor. Further, the red phosphor used for the display application is also required to have compatibility with color filters, so that the red phosphor having a broad (non-sharp) emission spectrum cannot be necessarily sufficient for the display application.
The red phosphor having a sharp emission spectrum includes red phosphors having Eu3+ or Mn4+ as a luminescent center. Among them, a fluoride phosphor obtained by dissolving Mn4+ in a fluoride crystal such as K2SiF6 to activate the fluoride crystal is efficiently excited by blue light and exhibits a sharp emission spectrum with a narrow half-width (Non-Patent Document 1). The fluoride phosphor can be used as a red phosphor to achieve an improved color rendering property and improved color reproducibility without lowering the luminance of the white LEDs. Therefore, in recent years, application of the fluoride phosphor to the white LEDs has been frequently studied.
On the other hand, the fluoride phosphor has problems that it has poor durability and the luminescent color changes over time. Therefore, it is proposed that a specific peak area ratio obtained by XPS measurement is controlled to improve the durability (Patent Document 1).